For instance, in bearings for use in automobile wheel, water tends to intrude into lubricants in the inside of the bearings under the effect of muddy water on road or rain water. Further, also in a case of bearings for use in guide rolls in continuous casting facilities or for use in backup rolls in rolling mills for iron and steel materials, cooling water or water for rolling tends to intrude in lubricants. Further, in bearings for use in dryer rolls for paper making machines, it is said that steams tend to intrude to the inside of the bearings since they are used in the process of drying moistened paper.
In the rolling bearings, it has been generally known that durability of lubricants is greatly deteriorated when water intrudes into them. For example, it has been reported that the rolling life of a bearing is deteriorated about from one/severals to one/twenties when 6% water content intrudes into the lubricant compared with that before intrusion of water (“The Surface-initiated and Subsurface-initiated Rolling Contact Fatigue”, NSK Bearing Journal No. 636, pp. 1-10, 1977, Furumura, Shirota, Hirakawa).
Further, it has been reported that when water intrudes into the inside of a bearing, for example, even when a slight amount of water content intrudes such as by about 100 ppm into the lubricants, the rolling fatigue strength of the bearing material is lowered as much as by 32 to 48% (“Effects of Water and Oxygen During Rolling Contact Lubrication”, Wear 12, pp 33-342, 1968, P. Schatzberg, I. M. Felsen).
Further as an example of J. A. Cirula, it has been reported for a test of intruding water in lubricants in a 4-ball rolling test that the bearing life was deteriorated to about {fraction (1/10)} compared with that before intrusion. In a rolling contact fatigue test for a bearing using steel balls charged with hydrogen, it has been shown that stainless steel balls have longer life than SUJ2 steel balls of second class bearing steel, but use of more expensive stainless steel balls than SUJ2 as a countermeasure for improving the bearings is practically difficult.
Then, in view of undesired effects of the water content on the bearing life, bearings have been sealed, for example, with contact rubber seals in order to prevent intrusion of water to the inside of rolling bearings used under a condition where water tends to intrude.
On the other hand, in bearings used under high rotational speed and high load such as bearings for use in automobile alternators, it has been reported a phenomenon that hydrogen is formed by decomposition of lubricants and the thus formed hydrogen atoms intrude into steels to cause peeling in an early stage as the phenomenon accompanied by the progress of remarkable increase in the speed of the equipments in recent years. For the early stage peeling phenomenon, it has been considered that water contained in lubricants (for example, water is contained always by about 0.1% in grease, or traction oils or ATF also contains water) is decomposed under high vibrations and high load, and hydrogen ions formed are adsorbed on the raceway surface and accumulated as hydrogen atoms to highly strained sites (in the vicinity of maximum shearing stress position) to cause stress corrosion cracking type peeling.
As the prior art intending for improving the working life of bearings used under high vibrations and high load accompanying such early peeling phenomenon, Japanese Patent Examined Publication No. 6-89783, for instance, discloses applying a heat treatment to bearing rings of grease-lubricated bearings in atmospheric air at a temperature of 200° C. or lower, to form an oxide layer (black coating) of 0.1 to 2.5 μm thickness on the rolling surfaces of the bearing rings to suppress intrusion of hydrogen to the bearing raceway surfaces caused by the decomposition of the lubricants.
Further, Japanese Utility Model Unexamined Publicatin No. 6-43349 also discloses a bearing in which an oxide layer of 0.1 to 2.5 μm thickness is formed to the rolling surface of the roll bearing lubricated with lubricants. Further, Japanese Patent Unexamined Publication No. 5-26244 discloses a bearing in which at least a fixed bearing ring is formed of 1.5 to 6.0% Cr steel in a grease-sealed bearing, so that brittle peeling can be prevented by passivated layers of the material per se of the bearing ring.
Further, “SAE Technical Paper: SAE 950944 (held on Feb. 27-Mar. 2, 1995”) discloses, in 1st to 14th paragraphs, a technique of preventing early peeling by analyzing fatigue mechanism of bearings for use in alternaters, replacing the sealed grease from E grease to M grease having a high damper effect, absorbing high vibrations and high load by the M grease while maintaining sufficient lubricant layers. For the early peeling phenomenon in bearings for use in engine auxiliaries, it is considered that if, for example, a grease with low damper effect is sealed in a bearing in a case where high vibrations/high loads exert by way of a belt from an engine crank to the bearing, lubricant layers are destroyed between rolling elements and raceway surfaces, hydrogen ions formed upon decomposition of water contained in the lubricants intrude through the fractured portions of the oil membranes and are absorbed to the raceway surfaces, to result in stress corrosion cracking type peeling.
A source of generating water in the bearings for use in engine auxiliaries is not restricted only to the water content in the grease. Since the bearings for use in the engine auxiliaries are often used in a high temperature state and they are cooled to an atmospheric temperature after stopping of the operation, it may be considered that water is formed by condensation from air present in slight spaces inside of the bearings or by intrusion of muddy water or the like from the outside.
In the sealing technique by using seals or the like as described above, it may be considered that the water content in the lubricants can be suppressed to about 10% or lower by the combined use of contact rubber seals attached to a chuck at the outside of the bearings and contact rubber seals incorporated in the bearing. However, there is a problem that intrusion of water into the lubricants can not be prevented completely. Even if intrusion of water from the outside can be prevented completely, it can not prevent water caused by condensation from air, for example, in the bearings for use in the engine auxiliaries which are stopped and cooled after operation at a high temperature.
On the other hand, the bearings disclosed in Japanese Patent Examined Publication No. 6-89783 and Japanese Utility Model Unexamined Publication No. 6-43349 intend to prevent intrusion of water into the inside of the bearings not by the rubber seal or the like but directly suppressing and preventing intrusion of hydrogen caused by the decomposition of the lubricants to the raceway surfaces of bearings. However, also this technique involves a problem in view of facilities or treating time since it is necessary to apply a laborious treatment of dipping a bearing ring in an aqueous solution of sodium hydroxide heated to a low temperature to form tri-ion tetroxide layer (generally referred to as black coating) and further it includes a treatment of etching the rolling surface in an oxidative aqueous solution such as nitric acid, hydrochloric acid or sulfuric acid as other solvent to such an extent as causing coloration. Further, since the thickness of the oxide layer is as large as 2.5 μm at the maximum, the bearing roughness is worsened when a large bearing load exerts, to possibly result in a problem of increase of vibrations.
On the other hand, it may be considered to heat the bearing material at high temperature in air as a means for forming the oxide layer to the bearing but, when the bearing is merely heated in air with no temperature control, oxidation occurs to form scales (black skins) of about several μm thickness on the surface of the material. Unevenness of the scales may lead to loss of metals and possibly form initiation points for pit or the like. Further, if the bearing is merely left in atmospheric air, it may be considered that moisture in air and steels may possibly take place reaction to cause atmospheric corrosion.
Further, for the bearing disclosed in Japanese Patent Unexamined Publication No. 5-26244, it is described that a chromium oxide layer is formed on the surface of the bearing ring to inactivate the rolling surface by the use of 1.5-6% Cr steels at least to the fixed ring to prevent intrusion of hydrogen caused by decomposition of grease. However, as shown in “Pretext for Japan Tribology Conference (Tokyo, 1995-5) pp 551-554”, since auto-slipping is caused at the inlet of the fixed rings in bearings for use in engine auxiliaries used under high vibrations/high load, the oxide layer intended for providing the damper effect is cut and, as a result, the outer ring in which early peeling occurs frequently undergoes load directly and it is actually difficult to prevent early peeling in the fixed ring.
The present invention has been accomplished for overcoming such disadvantages in the prior art and it is an object thereof to provide a surface-treated rolling bearing by forming an iron/chromium oxide series oxide layer of about 1 to 1000 nm thickness on the surface of bearing members used in a circumstance in which water is incorporated in lubricants, thereby enabling to suppress formation of hydrogen caused by decomposition of incorporated water and intrusion of hydrogen atoms into the matrix of the members and remarkably improve the bearing life, as well as a manufacturing method thereof.